Silicon, an important trace element, is essential for bone formation and other developmental processes. It may also be a factor in atheroschlerosis through its relations with hormones, cholesterol and other lipids, and its possible capacity to inhibit as well as promote calcification. Moreover, certain silicate minerals are pathogenic, implicated in silicosis, asbestosis and lung and other cancers. But though understanding of the biochemistry of silicon is therfore urgent, almost nothing is known other than what has been reported from diatom studies. We have shown in the diatom that Si is involved in a wide variety of metabolic processes and in a new system of biological mineralization, and that in both diatom and rat it is actively transported into mitochondria which contain both silicon granules. It is thought, on the basis of much research, that the mitochondria and the mitochondrial calcium phosphate granules must play a role in calcification. In view of the Si/Ca relationship, we are therefore focusing our studies more directly on the biochemistry of mineralization, particularly the role of Si in calcification and the relationship betweeen the Si and calcium phosphate granules. We propose therefore to characterize the silicate transport system and its carrier in mitochondria and certain cytoplasmic membrane vesicles, and determine the relationship, if any, between the calcium, phosphate, and Si transport systems. We propose, further, a detailed study of the Si granules; to determine their formation, ultra structure, mineral composition, and presence in other organelles; to identify and characterize their organic component(s) and determine the binding capacity; to follow the turnover of granules in the mitochondria; to determine whether Si is present in the calcium phosphate granules and if so, whether it results from intermingling of calcium phosphate and Si granules at the site or whether there is a third type of granule, i.e., Si-calcium phosphate. Studies will be carried out in diatom and rat, and in rabbit chondrocytes, in vivo and in vitro, using biochemical, electron microscopy, and electron-probe microanalytic methods.